Midsized exoplanets raise questions about solar system structure

Ars Technica : The arrangement of small rocky planets near the Sun and large gassy planets farther away in our solar system appears somewhat unusual based on our observations of exoplanets. Many of the planets that have been discovered have radii greater than that of Earth—the largest rocky planet—and smaller than that of Neptune—the smallest gas planet—and orbit much closer to their star. Scientists, however, have been able to measure the density of just 33 of those exoplanets. For the planets with a radius up to 1.5 times that of Earth, density increases regularly with size. Above that size, densities begin to vary widely, with some exoplanets having densities lower than those of smaller planets. The variation, the small sample size, and the fact that many of those planets orbit so close to their star that their atmosphere gets stripped means a clear picture of density distribution can’t be established. However, the measurements challenge the current model for gas planets’ formation and structure.